Many circuits such as current mirrors, input pairs of operational amplifiers and comparators require matched circuit components. The mismatch between transistors in these types of devices gets larger and larger as these types of devices get smaller and smaller.
A circuit diagram of a conventional 3 bit flash analog-to-digital converter (ADC) is shown in FIG. 1. The 3-bit flash ADC employs a resistor chain with 2N=8 resistors 1002 (N=number of bits) and 2N−1=7 comparators 1004. The reference voltage 1010 for each comparator 1004 is one least significant bit (LSB) lower than the reference voltage 1008 for the comparator immediately above it 1006. Each comparator produces a “1” when its analog input 1012 is higher than the reference voltage 1010 and produces a “0” when its analog input is lower than the reference voltage. For example, if the voltage of the analog input signal 1018 lies between the reference voltage on comparator C3 1014 and comparator C4 1016, comparators C1 1004 through C3 1014 produce “1” outputs and comparators C4 1016 through C7 1020 produce “0” outputs. The point where the code changes from “1”s to “0”s is the point where the input signal becomes smaller than the respective comparator reference voltage levels. An error may occur when there is an offset voltage between the reference voltage input 1010 and the analog input 1012 of a comparator 1004. When this occurs the comparator may output the wrong value if the offset voltage adds or subtracts sufficient voltage from the analog input signal to either raise it above or reduce it below the reference voltage.
SOI transistors are especially sensitive to variation due to heating effects because the buried oxide (BOX) upon which SOI transistors are constructed is an excellent thermal insulator. The threshold voltage (vt) of a typical transistor may change by as much as 1 mV per 1 degree change in temperature.